Stratosphere-Troposphere Exchange STE

As discussed in Chapter 2, the troposphere is characterized by decreasing temperature with altitude, whereas the opposite is true for the stratosphere. In the troposphere, vertical mixing occurs on a time scale 

FIGURE 12.2 UV absorption of O, at room temperature in the Hartley (200-300 nm) and Huggins (300-360 nm) bands (adapted from Daumont et at., 1992). 

As air is transported rapidly upward, for example in a convective system, cooling occurs (see Chapter 2), leading to the condensation of water as ice crystals. Because of this removal of water as moist tropospheric air rises, air in the stratosphere is very dry, of the order of a few ppm. Some water is also produced directly in the stratosphere from the oxidation of CH4 and H2. The so-called extratropical pump then moves the air poleward and downward at higher latitudes (Path I), warming the air as it descends. 

Using the terminology of Hoskins (1991), Holton and co-workers (1995) distinguish between the over- 

FIGURE 12.3 Schematic of wave-driven extratropical pump that drives global-scale transport from the tropical troposphere to the stratosphere and then poleward. Three possible paths for stratosphere-troposphere exchange (STE) are shown. (Adapted from Holton et al., 1995.) 

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